This invention relates to a new and useful improvement in precast manhole constructions of the drop manhole type.
A common type of manhole construction employs a base, a riser section and a tapered top section for supporting a cover frame which receives a removable manhole cover. The top section has an opening near the lower end thereof communicating with a pipe. The base has openings for connection to service lines.
The depth at which the service line is embedded in the ground determines the height of the inlet. Since service lines are generally buried only to the depth required in order to minimize excavation, the inlet is well above the manhole base in many installations. In order to protect workmen entering the manhole from fluid entering through the elevated inlet, it is customary to dam off a portion of the inlet and to install an outside drop. Such an outside drop employs a T-connection outside of the manhole assembly wherein one portion of the T is connected to the service line, a second portion of the T is connected to the inlet in the manhole, and a third portion of the T extends downwardly for connection to suitable outside conduit means encased in concrete and having an elbow directed into the manhole base adjacent to the bottom of the latter.
These outside drops have the disadvantage that they add considerable expense to the manhole construction, especially due to the fact that the drop assembly must be constructed at the job site. In addition, in the event of poor ground support and other factors, breaking of the encasing concrete and shearing of the conduit often takes place and leakage thus occurs.